Re: [PATCH 4/4] perf tools: Add dso__data_get/put_fd()

[Adrian Hunter <adrian.hunter@intel.com>]

On 20/05/2015 6:34 p.m., Namhyung Kim wrote:
> On Wed, May 20, 2015 at 11:33:09AM +0300, Adrian Hunter wrote:
>> On 20/05/15 09:34, Namhyung Kim wrote:
>>> Using dso__data_fd() in multi-thread environment is not safe since
>>> returned fd can be closed and/or reused anytime.  So convert it to the
>>> dso__data_get/put_fd() pair to protect the access with lock.
>>
>> This is good, but ideally dso__data_open_lock should be a rwlock.
>
> Agreed.  But as far as I can see, it might be a recursive mutex since
> it needs to allow to call dso__data_* functions while keeping fd open
> (ie. the dso__data_open_lock held).

Unless there are 'nolock' variants ;-)

>
>>
>>>
>>> The original dso__data_fd() is deprecated and kept only for testing.
>>
>> Maybe move it into perf/tests/dso-data.c since that seems to be the only caller.
>
> Okay.
>
>>
>>>
>>> Cc: Adrian Hunter <adrian.hunter@intel.com>
>>> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
>>> ---
>>>   tools/perf/util/dso.c              | 44 +++++++++++++++++++++++++++++---------
>>>   tools/perf/util/dso.h              |  9 ++++++--
>>>   tools/perf/util/unwind-libunwind.c | 38 +++++++++++++++++++-------------
>>>   3 files changed, 64 insertions(+), 27 deletions(-)
>>>
>>> diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
>>> index 21fae6908717..5227e41925c2 100644
>>> --- a/tools/perf/util/dso.c
>>> +++ b/tools/perf/util/dso.c
>>> @@ -471,27 +471,49 @@ static void try_to_open_dso(struct dso *dso, struct machine *machine)
>>>   }
>>>
>>>   /**
>>> - * dso__data_fd - Get dso's data file descriptor
>>> + * dso__data_get_fd - Get dso's data file descriptor
>>>    * @dso: dso object
>>>    * @machine: machine object
>>>    *
>>>    * External interface to find dso's file, open it and
>>> - * returns file descriptor.
>>> + * returns file descriptor.  Should be paired with
>>> + * dso__data_put_fd().
>>>    */
>>> -int dso__data_fd(struct dso *dso, struct machine *machine)
>>> +int dso__data_get_fd(struct dso *dso, struct machine *machine)
>>>   {
>>> +	pthread_mutex_lock(&dso__data_open_lock);
>>
>> I would check the return on all lock functions and consider failure to be an
>> error. i.e.
>>
>> 	if (pthread_mutex_lock(&dso__data_open_lock))
>> 		return -1;
>
> Ah, forgot to check the locking operation itself.  So do you suggest
> that we should check the return value of the locking in every place?

Sure. Could print an error too.

>
>
>>> +
>>>   	if (dso->data.status == DSO_DATA_STATUS_ERROR)
>>>   		return -1;
>>
>> The status check can be done before taking the lock.
>
> Right.  But I did it this way since I'd like to make sure to grab the
> lock unconditionally when calling the get() function.  See below.
>

Can change that though ;-)